Production of phosphate fertilizers and resultant product



1960 L. J. REVALLIER ET AL 2,948,602

PRODUCTION OF PHCSPHATE F'ER'IILLZERS AND RESULTANT PRODUCT Filed March1, 1957 Mol CaO/P O 1.7 Kouribgha, mol F/P 0 Q32 X Koia mol F/P 0 0.63 0Florida pebble, mol FIP EJ 0.83

nited States Patent "ice V "T- Patented'Aug; 9,1960

V omitting the measures according to the invention can be I p z 948 0explained on the assumption that, during evaporation of i theneutralized decomposition liquor, the water-soluble PRODUCTION OFPHOSPHATE iFERTILIZERS AND ammonium 'phosphat'epresent in the liquordecomposes RESULTANT PRODUCT into water-soluble monoammonium phosphate.

Leonardus J. Revallier and Willem Slot, both of Geleen, Netherlands,assignors to Stamicarbon N.V., Heerlen, Netherlands Filed Mar. 1, 1957,Ser. No. 643,267

Claims priority, application Netherlands Mar. 2, 1956 3 Claims. (CI.71-39) In British Patent No. 430,3 80, there is disclosed a process forthe production of a mixed fertilizer containing nitrogen, calcium andphosphate. According to that patent, the raw phosphate is decomposedwith nitric acid, ora mixture of nitric acid and another strong acidsuch as -phosphori c acid, and the aciddecomposition liquor iscontinuously neutralized with NH at a constant pH by above will yieldfertilizers the phosphate of which is almost completely soluble inammonium citrate, only if said continuous neutralization is effected atthe neutral point, i.e. if the pH is constantly maintained duringneutralization at 6.0 to 7.5.

The graphs illustrated in the accompanying drawing show the relationbetween the ammonium citrate-solubility of the phosphate precipitatedduring'neutralization and the pH maintained in the reservoirduringpartial or complete neutralization for three fluorine-containingvphosphates of quite different origin, namely Florida pebble phosphate,Kouribgha phosphate and Kola phosphate. From these graphs, it isapparent that, in order to achieve almost complete ammoniumcitrate-solubility of the productsobtained from thesefluorine-containing. phosphates, the neutralization must becarried outeither in a strongly acidic or in a perfectly neutral medium.

In addition, it was found that during the further processing of theneutralized decomposition liquor into a granulated fertilizer,considerable losses of NH, are incurred, particularly duringevaporation, which losses amount to more than 10% of the quantity of NH,added during neutralization.

Applicants have discovered that these NH losses can be avoided in asimple manner by adding to the solution which has been continuouslyneutralized at the neutral point, suflicient acid to bring the pH to avalue of approximately 4by which is meant a pH value of from 3 to4.5thereafter, the solution canbe further evaporated, granulated anddried.

The said addition of acid does not appreciably lower the ammoniumcitrate-solubility of the phosphate, and on the other hand, NH is nolonger set free during evaporation, granulation and drying.

However, if the fluorine-containing decomposition liquor is neutralizedat a pH of approximately 4 and then evaporated, then, there is no lossof NH but neither does the phosphate become completely soluble inammonium citrate. The loss of ammonia incurred when By X-ray examinationand analysis of the calcium phosphates formed during neutralization, ithas been found thatif the fluorine-containing decomposition liquor,after having beeriadjustedto a molar CaO/P O ratio of 21in thelnsualmanner-e.g. by crystallization and separation of calcium nitrate, orbyaddition of phosphoric acid-is continuously neutralized with NH at apH of 6.0 'or higher, dicalcium phosphate is not precipitated,rbut.acitrate-soluble.compoundiresembling apatite, which contains'oonsiderablyless P 0 per mole CaO than dicalcium phosphate. As a consequence, thesolution contains a quantity of P 0 which-cannot combine with CaO and,on. neutralization in a neutral medium, is converted into diammoniumphosphate. At the fairly high temperatures at which evaporation takesplace, namely 1 to 145 0., this diammonium phosphate is unstableandichanges', with evolution of'NH into monoammonium phosphate which isstable at said temperatures.

' The'above reactions may be'represented by the following equations,assuming that the raw phosphate used corresponds to the formula Cai IPOQF and that the calcium phosphate precipitated during neutralizationaftersubtraction of CaF -shows a CaO/P O ratio of 2.65.

According to the invention, the citrate-solubility of the calciumphosphate is not impaired if, prior to the evaporation, sufiicient acidis added to convert the diammonium phosphate, completely or for the mostpart, into monoammon-ium phosphate and ammonium nitrate, according tothe equation:

7 Hence, if the foregoingbe correct, then, the process of the presentinvention should yield a fertilizer which,

apart from approximately 21% total N (11.6% NH N and 9.4%nitrate-N),contains 22% P 0 which is almost completely citrate-soluble, and half ofwhich is water- This has been confirmed in actual practice as soluble.will be seen from the following example:

. Example 1,100 grams of Florida phosphate (50% CaO; 34.1% P 0 3.7% F)were decomposed with 2,200 grams 55% nitric acid. After completedecomposition, 1,180 grams Ca(NO -4 aq. were separated by cooling.

The resulting liquor was then treated with 237 grams NH at a pH of 6.7.By adding grams 55 nitric acid, the pH of the neutralized mass wasreduced to 4.0,

after which the mass was evaporated at C. The melt obtained was thengranulated and finally dried in a rotary drier. 1,700 grams of endproduct were obtained,

3 containing 20.1% N' (11.4% NH -N and 8.7% N N) and 20% P 0 (relativecitrate-so1ubility 96%, relative water-solubility 49%).

Obviously, potassium salts can be added to the decomposition liquor toobtain an N.P.K.-fertilizer.

We claim:

1. A process for producing from fluorine-containing raw calciumphosphate a mixed fertilizer comprising nitrogen, calcium and phosphate,wherein the phosphate is substantially completely soluble in ammonium.citrate and soluble to a large extent in water; comprising the steps.of: treating the raw phosphate. with nitric acid to produce an aciddecomposition liquor; adjusting the molar CaO/P O ratio of the liquor toa value not greater than 2; continuously neutralizing thethus adjustedliquor to a pH within the range of 6 to 7.5 by simultaneouslyintroducing streams of the adjusted liquor and NH into a contactingvessel; reducing the pH of theneutralized liquor to about 4, whereby theammonium-citrate solubility of the contained phosphate is. notappreciably lowered; evaporating the neutralized liquor of the reducedpH to obtain a melt; thereafter drying'said melt, and recovering thedried melt which forms the above mentioned mixed fertilizer. i

2. Process according to claim 1, wherein the pH of the neutralizedliquor is reduced by'the addition of nitric acid.

3. A granular mixed fertilizer containing nitrogen, calcium, phosphateand fluorine, said fertilizer ,produced from raw calcium phosphate by aprocms comprising the steps of: treating the raw phosphate with nitricacid to produce an acid decomposition liquor; adjusting the molar Cato/P0 ratio of the liquor to a value not greater than 2; continuouslyneutralizing the thus adjusted liquor to a pH within the range of 6 to7.5 by simultaneously introducing streams of the adjusted liquor and NHinto a contacting vessel; reducing the pH of the neutralized liquor toabout 4, whereby the ammonium-citrate solubility of the containedphosphate is not appreciably lowered; evaporating the neutralized liquorof the reduced pH to obtain a melt; thereafter drying said melt, andrecovering the dried melt which forms the above mentioned mixedfertilizer and which contains a complex of calcium phosphate and CaFwherein the phosphate is substantially completely soluble in ammoniumcitrate and soluble in water in excess of about and wherein the calciumphosphate after subtraction of CaF shows a CaO/P O ratio of about 2.65.

References Cited in the file of this patent UNITED STATES PATENTS1,876,501 Johnson Sept. 6, 1932 2,061,191 Foss et al Nov. 17, 19362,656,266 Calmeyer Oct. 20, 1953 2,707,676 Picot et al May 3, 19552,726,949 Andres et al Dec. 13, 1955 2,738,265 Nielsson Mar. 13, 1956819,955 Slot Jan. 14, 1958

1. A PROCESS FOR PRODUCING FROM FLUORINE-CONTAINING RAW CALCIUMPHOSPHATE A MIX FERTILIZER COMPRISING NITROGEN, CALCIUM AND PHOSPHATE,WHEREIN THE PHOSPHATE IS SUBSTANTIALLY COMPLETELY SOLUBLE IN AMMONIUMCITRATE AND SOLUBLE TO A LARGE EXTENT IN WATER, COMPRISING THE STEPS OF:TREATING THE RAW PHOSPHATE WITH NITRIC ACID TO PRODUCE AN ACIDDECOMPOSITION LIQUOR, ADJUSTING THE MOLAR CAO/P2O5 RATIO OF THE LIQUORTO A VALUE NOT GREATER THAN 2, CONTINUOUSLY NEUTRALIZING THE THUSADJUSTED LIQUOR TO A PH WITHIN THE RANGE OF 6 TO 7.5 BY SIMULTANEOUSLYINTRODUCING STREAMS OF THE ADJUSTED LIQUOR AND NH3 INTO A CONTACTINGVESSEL, REDUCING THE PH OF THE NEUTRALIZED LIQUOR TO ABOUT 4, WHEREBYTHE AMMONIUM-CITRATE SOLUBILITY OF THE CONTAINED PHOSPHATE IS NOTAPPRECIABLY LOWERED, EVAPORATING THE NEUTRALIZED LIQUOR OF THE REDUCEDPH TO OBTAIN A MELT, THEREAFTER DRYING SAID MELT, AND RECOVERING THEDRIED MELT WHICH FORMS THE ABOVE MENTIONED MIXED FERTILIZER.